1. Field of the Invention
The present invention generally relates to an improved high temperature combustion tube generally used within a Total Organic Carbon Analyzer. More specifically, the present invention provides for a combustion tube with distinct inert and catalytic chambers; as such, inert (energy-absorbing) materials are decoupled from catalytic materials.
2. Background Information
Conventionally, the oxidative furnaces utilized in high temperature carbon-oxidative combustion contains an inner assembly tube that is fabricated from quartz. This tube is in turn packed with various layers of materials that provide a profile to ensure combustion of the carbon within the furnace.
Typical packing of the tube starts furthest from the inlet region of the tube, where the tube is loaded with catalytically active materials (such as platinum on an alumina, zirconia, or titania substrate, or platinum as a solid pellet, or platinum as a porous pellet). Above the catalytically active materials, rests an inert section of quartz wool, upon which quartz chips or quartz beads are loaded.
A sample is introduced in the top via a small injection needle. Upon impact of the sample on the top layer of either the quartz or catalyst, the sample is vaporized, generating a significant expansion pulse or shock front. The energy of the expansion pulse is transferred in both the gas phase as well as by direct solid-to-solid contact between the quartz chips or beads to the very bottom of the reactor tube. This mode of energy transfer results in chipping or flaking of the platinum catalyst, decreasing the activity and subsequent efficiency of the platinum to convert the carbon in the sample to carbon dioxide. Moreover, the fine platinum, quartz, and platinum support particles that flake off are transferred by gravity and transport gas flow, out of the active (hot) reactive zone, down to the exit vent of the reaction tube. The vent section of the reactor tube is typically at temperatures below 200 C to allow conventional coupling of the reactor tube to Teflon or other inert tubing for transport to the bulk water condenser element and/or other high-efficiency drying element.
The purported reason for the upper quartz layer (either as quartz wool, or as quartz chips, or beads) is to absorb the shock of vaporization of water contained in the sample transferred for combustion to the reaction tube. However, since the quartz chips/beads are in direct (or after multiple injections come to be in direct) contact with the catalyst, the shock of expansion is directly coupled to the platinum catalyst. The resulting impact causes the catalyst to crack and flake off of the ceramic substrate. The platinum that flakes off, or the beads that have cracks or chips typically have reduced catalytic activity. Moreover, these catalyst particles (microscopic flakes of platinum) migrate to the bottom of the furnace tube. Since this region of the furnace tube is generally of reduced temperature, the “free” catalyst particles loose their effectiveness. These free catalyst particles inadvertently increase the back pressure of the system due to blockage of the exit vent.
Another effect that occurs upon addition of sample into the reactor tube is the deposit and transport of salts and other inorganic oxides onto the initial quartz body. With time, these salts migrate or are “channeled” by various means into the catalytic body immediately below the upper quartz or other ceramic elements in the upper packing layer. These salts or inorganic oxides coat the catalyst, severely limiting the catalyst from oxidizing the organic species present in the gaseous stream. As a function of time, these salts tend to increase the back pressure, and contribute to additional coupling of shock wave energy by direct contact with the platinum substrate (i.e. the inorganic salts and oxides further accelerate the deterioration of the platinum catalyst).
Finally, instruments known in the art require the user to make and break connections to the combustion tube from both the top and bottom of that tube. This makes servicing the instrument a very difficult procedure.
In view of the limitations of combustion tubes known in the art, a great need exists for improvement with respect to these tubes. Applicant's invention provides novel solutions to the problems mentioned above. By employment of a unique interior tube configuration, Applicant's invention provides a means to decouple the shock wave propagated by direct contact between the top-layer quartz (or other energy absorbing media) and the catalytic surface. Resulting advantages of the present invention include: (1) decreased rate of deterioration of the catalytic bed, (2) uniform thermal geometry in the base of the furnace and elimination of cold regions within the catalyst bed, (3) non transport of inorganic salts and inorganic oxides onto the surface of the catalyst, (4) easier servicing of the reactor tube, and (5) reduction in the amount of catalyst required.
Applicant's invention provides for uniform thermal geometry in the base of the tube. That is, the upper section of the tube is ‘cold’ and the bottom surface of the tube is maintained at the same temperature as the main body of the surrounding furnace. This ensures that moisture present in the system does not condense in the combustor tube after it enters the system. Consequently, there is no regeneration of a second shock wave as the superheated gas moves from the inlet (gas expansion side) into the outlet (catalytic reaction) side of the reactor tube. Moreover, because the gas is not allowed to cool, cold regions in the catalytic volume are not established. This greatly increases the efficiency of the reaction process. Summarily, in view of the prior art, the bifurcated-chamber design of the present invention permits less catalyst to be used—primarily because uniform thermal gradients maintain efficient reaction rates, and eliminate direct coupling of the expansion shock wave energy.
While incorporation of a bifurcated-chamber design may appear to be a subtle distinction at first glance, its effects completely change the operation and maintenance of these tubes and their overall combustion systems. For instance, prior art designs require excess loading of catalyst to account for decreased efficiency and catalyst degradation. However, the present invention eliminates the requirement to load the combustion tube with an excessive amount of catalyst.
The present system prevents degraded or dislodged inert particles, and inorganic salts and oxides deposited on the inert quartz body, from being transported to the catalyst side of the reactor tube. Rather, such particles are trapped along the bottom surface of the tube. As such, these unwanted particles cannot obstruct gas flow or attach to the catalyst materials. Again, as the catalyst remains free from interfering matter, its working efficiency is preserved.
The present invention allows complete service of the combustion tube from only the top-side, making replacement or service of the combustion tube more “user” friendly. Since the platinum catalyst does not rapidly degrade, servicing of the combustion tube allows the user to reuse the platinum catalyst. This feature alone results in a substantial reduction in the cost of servicing the combustion system.
Finally, the present invention eliminates the requirement of humidification of the oxidant. In prior art designs, the catalyst limits the sorption of carbon dioxide (or degradation products containing carbon). The sorption is due to the degradation of the catalyst—caused by exposure to inorganic salts and/or oxides and the resulting “barrier” coating of the catalyst, exposure of chemically active sites, or crevices and fissures within the support phase of the catalyst. In the prior art, humidification of the catalyst serves to hydrate the inorganic salts and/or oxides, converting them into a form in which the carbon dioxide or degradation products of the oxidation process are not as strongly sorbed. As such, devices in the prior art are much more complex with regard to plumbing and servicing, as humidification reservoirs are required. Applicant's invention, however, presents a very straightforward plumbing mechanism and eliminates the need for a humidification reservoirs.